The present invention relates to a display panel as represented by a liquid crystal cell, more specifically to a technology suitable for obtaining a narrow-frame liquid crystal display panel.
Liquid crystal display devices are remarkably widespread as image display devices for personal computers and various other monitors. Typical liquid crystal display devices of this kind include a backlight, which is a planar light source for illumination, that is provided on a back side of the liquid crystal cell such that the backlight irradiates a liquid crystal layer having predetermined expansion to even brightness as a whole, whereby images formed on the liquid crystal layer are visualized.
The liquid crystal cell is comprised of two glass substrates being laminated together via the liquid crystal layer. Surrounding portions of the two glass substrates are sealed with a sealing material, and a liquid crystal material is filled into a space between the two glass substrates and surrounded by the sealing material, whereby a liquid crystal layer is formed. While a part of the liquid crystal cell surrounded by the sealing material constitutes a display area, the part provided with the sealing material does not constitute the display area, but instead is referred to as a frame. Because it is desirable to obtain a wide display area for the resulting structure, the width of the frame should be made as narrow as possible, but still able to function as required.
FIGS. 16(a) to 16(c) are views showing a conventional liquid cell, wherein FIG. 16(a) is a plan view thereof; FIG. 16(b) is a cross-sectional view taken along line A-A in FIG. 16(a); and FIG. 16(c) is an enlarged, partial view of a corner portion of the liquid crystal cell in FIGS. 16(a) and 16(b).
In FIGS. 16(a) to 16(c), a first substrate 110 is shown and consists of a flat hard glass. The major part of the top surface of first substrate 110 is occupied by a display area 113 having a colored layer composed of color filters (CF) of red, green and blue (RGB) formed therein. A sealing material 130 is applied thereto along the perimeter of display area 113 of first substrate 110. Small spacers 40 are positioned over display area 113 in order to maintain a uniform thickness of the liquid crystal cell over the whole surface. A second substrate 120 is also shown, and also consists of a hard glass. It is provided with thin-film transistors (TFTs) on its surface opposite to first substrate 110. When second substrate 120 is stacked on first substrate 110, the applied sealing material 130 is pressed to adhere the two substrates 110 and 120 together. When these two substrates are adhered together, both are then subjected to a baking step (a thermal treatment), such that sealing material (e.g. epoxy resin) 130 is cured, whereby first substrate 110 and second substrate 120 are finally sealed together to form the liquid crystal cell. A liquid crystal material is filled into the liquid crystal cell thus formed, through an insertion port formed within sealing material 130, whereby an end product now results. The sealing material is then finally sealed.
In FIG. 16(c), sealing material 130 is applied as illustrated by solid lines 138. However, when the sealing material is pressed by laminating the first and second substrates, it spreads as illustrated by dotted lines 139. Here, when sealing material 130 spreads toward the inside of first substrate 110, it may penetrate display area 113 partially. If it does penetrate as such, it may cause display defects on the corners of display area 113. To avoid such penetration, it is necessary to distance the corners of display area 113 from an inner edge of sealing material 130. As such, the space between the display area 113 and the inner edge of sealing material 130 must be relatively broad. Therefore, the outermost edges of display area 113 are limited and not readily expandable if a larger area is desired.
Various modes of attempts to achieve frame narrowing by controlling the behavior of the applied sealing materials are defined in Japanese Patent Laid-Open Publication No. 2000-193989 (and its USA counterpart U.S. Pat. No. 6,317,186 B1), both assigned to the same assignee as the present invention. These modes inhibit sealing material spread at corner portions of the liquid crystal cell. According to conventional methods (prior to those taught in Japanese Patent Laid-Open Publication No. 2000-193989 and U.S. Pat. No. 6,317,186 B1), a sealing material applied at a corner portion of a liquid crystal cell tended to spread inward, i.e. toward the display area, during laminating of the two glass substrates, and thus the display area was reduced. The various modes defined in Japanese Patent Laid-Open Publication No. 2000-193989 and U.S. Pat. No. 6,317,186 B1, are intended to prevent such spread.
One such mode is illustrated in FIG. 17. Here, protruding (upstanding or upraised) portions 140 are provided on a glass substrate cell except at the corner portions thereof. These are along the top and side (as well as the bottom and opposing side, not shown) edges, as seen. As a result, a step is formed in the vicinity of the perimeter of the second substrate (120 in FIG. 16(a)) in a manner that upper surfaces of the corner portions are lower than the upper surfaces of protruding portions 140. Sealing material 130 applied in the vicinity of the perimeters of the second substrate show less height at the corner portion, which is lower by the height of protruding portion 140. Sealing material 130 at the corner portion is pressed less than the sealing material at protruding portion 140 during substrate lamination, by the amount equivalent to the height of the protruding portion. On the other hand, sealing material 130 applied over portions 140 is more pressed by the amount equivalent to the volume of the respective protruding portion. In other words, this mode modifies the inside edge of sealing material 130 at the corner portion from an arc (FIG. 16(c)) to an approximate right angle by using this difference of pressed sealing material at the corner portion versus that in other regions (sides).
Another mode defined in Japanese Patent Laid-Open Publication No. 2000-193989 and U.S. Pat. No. 6,317,186 B1 is illustrated in FIG. 18. This is an example of providing a protruding (upstanding or upraised) portion 150 having rectangular planes at the corner portions of second substrate 120, so that a pressed amount of sealing material 130 is directed outwardly (from the display area 113) toward the substrate's corners. Portion 150 is formed at the corner portion to coincide with a diagonal through the center of the liquid crystal cell. Sealing material 130 on an upper surface of the protruding portion is more pressed by second substrate 120. Since sealing material 130 is applied thereto in a shape as illustrated in FIG. 18, the inside edge of the sealing material at the corner forms an approximate right angle, due to the increase in pressure on the sealing material 130 at protruding portion 150.
When a sealing material is applied with a dispenser, the traveling speed of the dispenser along the outer regions of the substrates needs to be slowed down at the corner portions. Assuming that an amount of sealant discharge per unit of time is maintained constant, the amount of sealing material applied in the corner portion is increased. As a consequence, although the mode shown in FIG. 17 provides certain advantages, further improvement is required in order to sufficiently inhibit sealing material 130 at the corner portion from spreading inward. In the mode shown in FIG. 18, the dispenser must move along a special trajectory at the corner portions during application of sealing material. That is, the trajectory of the dispenser becomes longer and more complex than the example shown in FIG. 17. This added complexity is, obviously, not desirable from a manufacturing standpoint.
Japanese Published Unexamined Patent Application 4-20929 discloses a liquid crystal cell in which an uneven portion is formed along peripheral portion of each of two substrates, a sealing material is applied on the uneven portion, and the two substrates are faced to each other and sealed. By providing the uneven portion on the portions on which the sealing material is applied, a strong adhesive strength is obtained, a penetration of water component into the liquid crystal cell is suppressed, and a reliability of a display panel is guaranteed.
Japanese Published Unexamined Patent Application 52-45947 discloses a technology for forming a groove along a peripheral edge of one of glass substrates of a liquid crystal cell. In a process for sealing the two glass substrates, an excess sealing material flows into the groove, so that a straight edge of the sealing material facing to a display area without an undesired wave like shape is realized. Since the edge of the sealing material facing to the display area is a straight line, a product value of the liquid crystal display panel is increased.
These documents, however, do not teach an improved method for applying the sealing material onto the corner regions of the substrate.
It is believed, therefore, that a method which assures precise sealing material spread relative to the display (viewable) area of such a cell in an improved manner over methods described hereinabove would represent a significant advancement in the art.